1. What happened
ParticlePhysicsExtensions.GetCollisionEvents returns larger than actual number of events. Issue is relevant for Editor only and only if particle burst occurs at the beginning of PS life. And if more than 1 collider exists.
Overall the issue is more complex and not always reproducible with the above conditions. Originally attached project with other recent fixes now works OK.

2. How we can reproduce it using the example you attached
Use project '4b9wq6ut.9qo With Spaces.zip' attached to one of the comments to this case.
- Open scene '000-startup'
- Run
---> Observe console output - (it'll print 'current particles count' every frame to show that actual number of particles is OK) and then will output final 'Test Result' message that prints smth like '...at line 71 expected: 50 was: 208...'

Reproducible also in 5.2.3, 5.3.0.